Separation of heavier hydrocarbons from natural gas

ABSTRACT

A method for separating heavier hydrocarbon components of a normally gaseous mixture, e.g., natural gas, from a relatively more volatile component, e.g., methane is accomplished by cooling the gas under pressure to effect condensation, adding to the cooled feed a small amount of heavier hydrocarbons, flash separating the mixture, e.g., in a first flash drum, adding to the separated gas phase a small amount of heavier hydrocarbons, and again flash separating the mixture, e.g., in an auxiliary flash drum. Desirably, the heavier hydrocarbons added to the feed gas before the first separation may be the liquid recovered from the auxiliary flash separation. The liquid recovered from the first and auxiliary flash separations is desirably chilled and subjected to a second flash separation at lower pressure to volatilize most of the lighter component, e.g., methane, recovered in the liquid with the heavier components. The liquid resulting from the second flash separation may then be fractionated into such components as ethane, propane, butane and gasoline. The methane-containing gasiform phases from the auxiliary and second flash separations are desirably combined with the fractionated ethane and supplied as the treated natural gas product.

United States Patent [72] Inventor AlbertStrum Flushing, NY. [21] AppLNo. 790,211 [22] Filed Jan. 10,1969 [45] Patented Nov. 23, 1971 [73] Assignee Hydrocarbon Research, Inc.

New York, N.Y.

[ 54] SEPARATION OF HEAVIER HYDROCARBONS FROM NATURAL GAS 3 Claims, 3 Drawing Figs.

[52] U.S.Cl 208/340, 55/88, 208/341 51 nn.c| C10g5/00 [50] FieldotSearch 208/340, 34l;55/88 [56] References Cited UNITED STATES PATENTS 2,222,275 11/1940 Babcock 208/340 2,805,979 9/1957 Vermilion 208/340 2,973,834 3/1961 Cicalese 55/85 3,054,745 9/1962 Forbesetal. 208/340 3,313,724 4/1967 208/340 Kniel Expander Primary Examiner-Herbert Levine Attorney-Nathaniel Ely ABSTRACT: A method for separating heavier hydrocarbon components of a normally gaseous mixture, e.g., natural gas, from a relatively more volatile component, e.g., methane is accomplished by cooling the gas under pressure to effect condensation, adding to the cooled feed a small amount of heavier hydrocarbons, flash separating the mixture, e.g., in a first flash drum, adding to the separated gas phase a small amount of heavier hydrocarbons, and again flash separating the mixture, e.g., in an auxiliary flash drum. Desirably, the heavier hydrocarbons added to the feed gas before the first separation may be the liquid recovered from the auxiliary flash separation. The liquid recovered from the first and auxiliary flash separations is desirably chilled and subjected to a second flash separation at lower pressure to volatilize most of the lighter component, e.g., methane, recovered in the liquid with the heavier components. The liquid resulting from the second flash separation may then be fractionated into such components as ethane, propane, butane and gasoline. The methane-containing gasiform phases from the auxiliary and second flash separations are desirably combined with the fractionated ethane and supplied as the treated natural gas 54 Deethunizer PATENTEDNHV 2 IHYI 3. 622 504 sum 1 or 3 INVENTOR. Albert Strum by W bmucoaxm PATENTEDuuv 23 e971 SHEET 2 [IF 3 .avcoaxm mmT INVENTOR. Albert Strum & AT TO R5? SEPARATION OF HEAVIER HYDROCARBONS FROM NATURAL GAS BACKGROUND OF THE INVENTION This invention relates to an improved process for the recovery of relatively less volatile hydrocarbons from a normally gaseous mixture consisting essentially of hydrocarbons e.g., natural gas. More particularly, it relates to the separation of a relatively more volatile hydrocarbon component e.g., methane, from such a mixture.

Natural gas, after the separation of entrained liquid, is composed mainly of methane. However, it usually also contains substantial proportions of heavier compounds, e.g., ethane, propane, butane and hydrocarbons boiling in the gasoline range. Hydrocarbons of higher molecular weight than methane have various uses which make it uneconomical to burn them with the methane as ordinary fuel gas. For example, propane is widely used in the production of ethylene and as a liquid petroleum gas for home use. Butane is commonly dehydrogenated to butylene and butadiene which are used in the production of synthetic rubbers and resins. Gasoline, of course, is much more profitably used as a fuel in internal combustion engines than in furnaces and burners.

The present invention is an improvement in the process disclosed in U.S. Pat. No. 2,973,834, issued Mar. 7,1961 to Michael J. Cicalese for Hydrocarbon Recovery from Natural Gas and assigned to the assignee hereof. The process of the Cicalese patent involves the separation of heavier hydrocarbons, i.e., hydrocarbons having a molecular weight greater than that of ethane, from natural gas by condensing such hydrocarbons at high pressure and low temperature and distilling the condensate. Because a substantial proportion of methane is condensed with the heavier hydrocarbons, the initial condensate is first subjected to a first flash separation to remove much of the methane from the condensate. Inasmuch as a significant proportion of heavier hydrocarbons tends to vaporize with the methane when subjected to flash separation, a small quantity of heavier hydrocarbons, desirably recycled from subsequent fractionation steps, is added to the condensed feed before the first flash step to improve the efficiency of separation.

The gasiform phase resulting from the first flash separation consists mostly of methane and some of the heavier hydrocarbons. This gas is then recompressed and sent to the output pipeline for treated natural gas. The heavier hydrocarbons in this phase are lost to further recovery steps. The liquid recovered in the first flash separation is subjected to a second flash separation at lower pressure and temperature to volatilize most of the remaining methane. The gasiform phase of the second flashing is combined as output with that from the first flashing, and the remaining liquid is then separated into its components by fractional distillation.

Although this process is generally satisfactory, further improvement in the separation of heavier hydrocarbons from the methane and ethane of natural gas would be highly desirable. Loss of heavier hydrocarbons in the gas phase of the first flashing should be held to an absolute minimum. However, the cost added by any additional process steps must be minimal as well, to make it economically feasible to employ any such improved separation process.

SUMMARY OF THE INVENTION It has now been found that separation of heavier hydrocarbons from a more volatile component, e.g., methane, may be substantially improved with only a minimal increase in the overall cost of processing.

More specifically, the addition of a heavier hydrocarbon fraction to each of two successive flash separations provides greatly improved separation, even at higher temperatures and pressures. Recovery of heavier hydrocarbons is especially good when the first addition of heavier hydrocarbon fraction to the feed comprises the condensate of a subsequent, or auxiliary, flash separation step. In such a preferred embodiment are obtained with an exceptionally low additional capital expenditure. Only an additional flash drum is required; no elaborate contacting tower or distillation apparatus is needed to obtain the improved separation of the present invention.

Furthermore, the flash steps may be carried out at pressure somewhat higher than the critical pressure of methane and at correspondingly higher temperatures. Therefore, the present process has lower refrigeration and recompression costs than previously employed processes carried out at or near the critical pressure of methane and at lower temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowsheet illustrating the process of the present invention.

FIGS. 2 and 3 are schematic flowsheets illustrating other modified forms of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawing, one preferred embodiment of the present invention shown in FIG. 1 will now be described in greater detail. All temperatures are in degrees Fahrenheit.

Natural gas feed at a pipeline pressure of 1050 p.s.i.g. and a temperature of enters the system at inlet 10, is expanded to about 800 p.s.i.g. in expander 12 and then is cooled in heat exchanger 14 to about 65 by returning product gas in lines 16 and 18. The cooled feed gas in line 15 from heat exchanger 14 is then dried in a drier 20 which is packed with a suitable desiccant such as alumina. The dried gas leaves the drier 20 by line 22 and passes through a low temperature heat exchanger 24. Cooling in the low temperature heat exchanger 24 is preferably effected by returning product gas in lines 16 and 18, which is at a lower temperature than in exchanger 14, and also by cascade refrigeration with successive externally cooled refrigerant streams. For example, the refrigerant streams may comprise boiling propane (C at 15 circulated through line 26 at the warmer end of exchanger 24, or other refrigerants circulated through suitable lines at other points in exchanger 24.

The heat exchangers 14 and 24 may, of course, comprise a battery of several heat exchangers in various positions and with various streams in heat exchange relationship with each other. The specific arrangement of such exchangers will depend on such engineering considerations as minimizing capital costs or operating costs.

The partially condensed gas from line 22 leaving heat exchanger 24 is combined with a substantially paraffinic hydrocarbon stream from line 34, obtained as hereinafter described, and in then passed to a first flash separator, or drum 36, wherein it is separated at about 50 and 800 p.s.i.g. into a condensate and a gas comprising methane, free of a substantial portion of the heavier hydrocarbons originally present in the natural gas. The condensate, containing most of the hydrocarbons heavier than methane originally present in the natural gas, is drawn from the flash separator 36 by line 38 for further processing, and the thus separated gas passes out through line 40.

The gas in line 40 is composed of methane and a certain amount of heavier hydrocarbons originally present in the natural gas. Although it is desired to separate out all of these hydrocarbons with the condensate in flash separator 36 and pass them through line 38 for further processing, a portion of these hydrocarbons instead tends to vaporize with the methane. In accordance with the prior art, any heavier hydrocarbons vaporized with the methane in the first flash I separator 36 would be passed out of the system as part of the treated natural gas and would be lost to any possibility of further recovery. However, according to the present invention, the gas in line 40 is subjected to a further separation step, as described below, so that an additional fraction of the heavier hydrocarbons is recovered from the natural gas and subjected to further fractionation.

More particularly, the gas in line 40 is combined with a liquid stream of heavier hydrocarbons from line 42. The stream carried by line 42 desirably contains a portion of heavier components obtained from hydrocarbon fractionation as indicated hereinbelow all of which has been cooled in heat exchanger 43. In general, the stream from line 42 may be any hydrocarbon fraction having an average molecular weight greater than that of butane. However, in most cases, at least a major proportion of the hydrocarbons used will have a boiling point not higher than the kerosene range, i.e., 500. A hydrocarbon fraction most of which boils in the gasoline range is preferred. Usually, the amount of hydrocarbon added through line 42 to the gas of line 40 may range from about 0.5 to mole percent of the amount of feed gas flowing through line 22.

The mixture of hydrocarbons from lines 40 and 42 is then passed for further cooling in heat exchanger 41 and then into an auxiliary flash separator 44 and separated into a vapor phase and a condensate at about 60 and a pressure of 800 p.s.i.g. The gas phase, which consists mostly of methane, is passed out of the system as treated natural gas through line 18. Line 18 carries the gas through heat exchangers 24 and 14 wherein it is warmed by the incoming feed gas passing through lines 22 and 10, respectively. The gas from line 18 is then recompressed to a pipeline pressure of about 970 p.s.i.g. by compressor 46 and passed out of the system as treated product through line 48.

The liquid phase from the auxiliary flash separator 44 is drawn off through line 34, combined with the feed gas in line 22, and passed into the first flash separator 36, as hereinabove described. Thus, according to the preferred embodiment of the present invention, some of the heavier hydrocarbons which have originally vaporized in the first flash separator 36 are caused to recycle therethrough by subsequent separation in'the auxiliary flash separator 44, thereby enhancing the recovery of heavier hydrocarbons.

As mentioned hereinabove, the liquid phase obtained in the first flash separator 36 is drawn off through line 38. The pressure of the liquid from separator 36 is reduced through valve 46 resulting in a further cooling to about -60 due to Joule- Thompson expansion, and the resulting vapor liquid mixture is passed by line'38 through the cold end of heat exchanger 24 wherein it serves to cool incoming gas. It is then separated at 50 and 500 p.s.i.g. in the second flash separator 49 yielding a gas stream, again composed mostly of methane, and a liquid containing most of the hydrocarbons heavier than methane in the original gas and still containing some methane. The condensate in the second flash separator 49 is passed by line 54 through the colder portion of low temperature heat exchanger 24 and into a deethanizing column 56. The gas stream from the separator 49 passes through lines 50 and 16 in the heat exchangers 24 and 24. Then, after the gas in line 16 leaves heat exchanger 14, it is compressed by compressor 52 to the desired pressure and is combined with the gas in line 18 and fed into the product line 48.

As mentioned above, the liquid from the second flash drum 49 is fractionated in deethanizer 56 into an overhead product consisting almost entirely of methane and ethane and a bottoms product containing propane and heavier hydrocarbons. Heat for column 56 is provided by steam in a reboiler 58 and reflux by boiling ethane at -65 in a condenser 60 at the top, the ethane passing through the condenser 60 in line 62. The overhead gas at 49 and 460 p.s.i.g. leaves the condenser 60 by line 16 and is combined with outgoing gas from line 50. In this particular embodiment, ethane is not separated for further use but is left in the product gas. However, it is within the scope of this invention to further separate ethane from the methane/ethane mixture.

The deethanizer bottoms product is transferred by line 64 to further fractionation and treatment stages 66 which are well known to those skilled in hydrocarbon fractionation. For example, the fractionation stages 66 may include a debutanizing column, caustic wash tower, and depropanizing column to separate out propane and butane and to supply the separated fractions through lines 68 and 70, respectively. A recycle fraction, described hereinabove, is delivered from the treatment and fractionation stages through line 42, and gasoline, which may be the same as the recycle fraction, is delivered through line 72.

FIG. 2 is a diagrammatic representation of an alternate mode of operation which results in even greater increased methane recovery and purity over that of FIG. 1. In this mode, feed at is cooled by heat exchange with product gases in heat exchanger 114 and the cooled gas 115 is then dried at 120. The cold dry gas at 122 is then further cooled in heat exchanger 124 and introduced into flash tank 123. The vapors from flash 123 are suitably expanded in cold expander 125 and mixed with the condensate which has been expanded. Through valve 125a with the combined stream in line 127 now passed into first flash separator 136. As in the earlier fonn, the gas stream, line 127, is first mixed with a heavy hydrocarbon fraction 1.34. The mixture is flashed at a temperature of about 50 and a pressure of about 785 p.s.i.g. to produce a vapor overhead at line 140.

This vapor is mixed with a heavier hydrocarbon fraction 142 from the final fractionation step 166 which has been cooled in heat exchanger 143. The mixture is further cooled in heat exchanger 141 and introduced to drum 144, wherein it is flashed at a temperature of about 50 and a pressure of about 785 p.s.i.g. The vaporous overhead from this flash step in line 118 is passed through heat exchangers 124 and 114 and is compressed at 146 and introduced to the methane product line 148.

The condensate bottoms 134 from flash drum 144 represents the heavy hydrocarbon fraction which is added to the cooled feed in line 127.

The condensate from flash drum 136 is introduced through line 138 and valve 146 to separator drum 149 after passing in heat exchange at 124 with the above mentioned vapor stream 1 18.

This condensate is flashed in drum 149 at a temperature of about 50 and a pressure of about 385 p.s.i.g. The vapor overhead from this flash step is removed in line 116, warmed in exchangers 124 and 114 and then compressed in compres sor 152 and introduced through line 118 to the product gas line 148. The condensate bottoms are removed through line 150 and valve 152 and is then passed to final drum 156.

The material is flashed in drum 156 at a temperature of about 50 and a pressure of about p.s.i.g. The vapor overhead 163 from flash drum 156 is combined with vapors in line 161 and pass in line 165, compressed in compressor 152a and thereafter combined with the vapor overheads from the previous flash stages. The heavy condensate bottoms from drum 156 is removed through line 157 and introduced to stripper 15 9. Any methane still present in the condensate is vaporized and removed through line 161 and then introduced to the vapor overhead line 163. The heat is supplied to stripper 159 by the reboiler consisting of the bypass line 164 and reboiler 158. The heavy bottoms material which now includes most of the C +hydrocarbons which were initially present in the feed is introduced to the fractionator 166 as heretofore described. The side streams 167, 168, I70 and 172 correspond to the similar streams from fractionator 66 in FIG. 1. The heavy hydrocarbon is removed at 142.

FIG. 3 represents still another mode of operation which is particularly suitable in cases where high ethane and/or propane recoveries are desired. In this case, the stripper 159 in FIG. 2 is replaced by two drums 256 and 260. Vapors from the first drum 256 are contacted in the second drum 260 with the heavier hydrocarbon fraction 242 which was previously sent directly to drum 44 in FIG. 1 and drum 144 in FlG. 2. This contact reduces the loss of ethane and/or propane in the low pressure gas leaving the plant, by absorbing these constituents in the heavier hydrocarbon fraction. The liquid from this drum is then sent as before to drum 244.

While each of the elements in FIG. 3 have not been described in detail, they correspond to the elements in FIG. 2 merely changing the subscript by 100 i.e., the feed is indicated at 210 in P16. 3, whereas it appears as l in FIG. 2.

It is believed that the improved separation of the present process may be explained as follows. The addition of heavier hydrocarbon components to a natural gas mixture raises the convergence pressure of the resulting mixture. As understood by those skilled in the art, the convergence pressure of a given mixture is the pressure at which the gas-liquid equilibrium constant of each component of the mixture is equal to unity. Thus, at the convergence pressure of a given mixture, the gas and liquid phases produced by flash separation would have identical compositions because the relative volatilities of all components would be the same. Conversely, as the composition of a mixture is changed so that its convergence pressure is raised, then flash separation of the mixture at a given pressure results in better separation of the components, the relative volatilities of the components being further apart.

EXAMPLES The invention will now be further illustrated by typical examples.

Natural gas in treated to remove a substantial portion of the hydrocarbons heavier than methane according to the process described hereinabove. The composition of the feed gas is shown in table 1. Carbon dioxide and hydrogen sulfide which is present in the original gas are removed by conventional means to produce the feed gas illustrated.

TABLE 1 Mole Percent it will be seen that present two-step process gave superior separation over the one-step process of the Cicalese patent. These results are surprising inasmuch as the seemingly simple variation of the present invention would hardly be expected to 5 increase butane recovery from 2,391 barrel/day (control) to 2,516 barrel/day (example -4 and to reduce pentane-andabove loss from 36 barrel/day (control) to 27 barrel/day (example 4 especially while operating at higher temperatures, thereby decreasing refrigeration costs as well as reducing 10 recompression costs. The conditions of example 4 gave the best results, and thus are the conditions described hereinabove with respect to the preferred embodiment of the process of the present invention. Of course, it will be apparent from the data that other temperatures and pressures may alperior operations, the ranges of temperatures will ordinarily run between about 40 and aboutl00. ln a similar manner, depending on the feed and depending on the requirements of the user, the ranges of pressure will ordinarily run between about 500 p.s.i. and 1,100 p.s.i.

Variations may, of course, be made without departing from the spirit and scope of the present invention.

Having thus described the invention, what it is desired to 25 claim and thereby secure by Letters Patent is:

35 separating said natural gas mixture into a gasiform phase and a condensate phase,

adding to said gasiform phase a second mixture of said higher molecular weight components to form a final combined mixture, and

separating said final combined mixture at about 60 F. and 800 p.s.i.g. into a second gasiform phase and a second condensate phase. 1

2. The process of claim 1, wherein said second condensate Nitrogen 0.76

Melba" 39-62 phase is said first mixture of said higher molecular weight Ethane 5.18

Propane 2.34 components iso-Butane 0.60 3. A process for the separation of methane from methane P' containing natural gas having higher molecular weight comiso-Pentane 0.27

hum: ponents than methane which comprises the steps of.

017 so a. cooling said natural gas to a temperature between about Heptanc 0.14 100 and about -40 F. at a pressure between about 500 g and about 1,100 p.s.i.g.;

Illln l i H nviei' hydrocarbons 1:. adding to said natural gas a first mixture of said higher hniling tWci 400 F. None molecular weight components;

c. separating said natural gas mixture into a gasiform phase and a condensate phase;

Table 2 Presents data mdlcafmg the results l P of d. adding to said gasiform phase a second mixture of said the present invention as carried out under varying pressure hi h molecular i h components to f fi lc and temperature conditions. For purposes of the present exbi d i d pl the r y heavier hydrocarbons were simulated y e. separating said final combined mixture at between about a mixture of 2.18 percent heptane and 1.52 percent toluene by 100 and -40 F. and at between about 500 and 1,100 moles, taking the composition of the feed gas as 100 percent. p.s.i.g. into a second gasiform phase and a second con- Thus, the total heavier hydrocarbons recycled is taken as 3.70 densate phase; percent by moles for illustration purposes. f. recovering a methane containing product.

TABLE 2 Example Control 1 2 3 1 4 5 6 First drum F 70 60 60 50 -50 5() Auxiliary drum F 70 60 70 -60 li0 50 Pressure, p.s.i.a 800 800 800 800 800 1, 000 First drum liquid, mole percent condensed:

Methane 14. 2 15. 3 11. 9 1). 8 11. 7 9. 6 13,7

Ethane. 61. 5 62. 2 54. 8 50. 2 48. 0 46. 6 49. 1

Propane. 80. 5 92. 7 00. 9 88. 4 86. 9 84. 8 81 Butane 03. 3 99. 1 9s. 8 118.4 98.2 07. 7

Pentano 98. 0 99. J 99. 11 9!). 8 ill). 8 119. 8 99, 2

Hexane and above U18. 0 96- 5 116.6 116. (l 116. 7 U6. 2 8i). 2 Auxiliary drum vapor, pound-mole hr.:

Flash gas, mole/hr 11698.8 17, 263. 6 17,867-1 18, 5011. 9 18,508 8 is, 614. 5 17,811). 2

Million std, cub. It./day 161 157 163- 5 168- 6 181) 109. 6 162 Pentane and above, loss, barrel/day. 38 28 28 27 27 31 96 ternatively be employed with similarly good results. For su-. 

2. The process of claim 1, wherein said second condensate phase is said first mixture of said higher molecular weight components.
 3. A process for the separation of methane from methane containing natural gas having higher molecular weight components than methane which comprises the steps of: a. cooling said natural gas to a temperature between about -100* and about -40* F. at a pressure between about 500 and about 1,100 p.s.i.g.; b. adding to said natural gas a first mixture of said higher molecular weight components; c. separating said natural gas mixture into a gasiform phase and a condensate phase; d. adding to said gasiform phase a second mixture of said higher molecular weight components to form a final combined mixture; and e. separating said final combined mixture at between about -100* and -40* F. and at between about 500 and 1,100 p.s.i.g. into a second gasiform phase and a second condensate phase; f. recovering a methane containing product. 